The objective of this study is to determine the role of immunophilins in the regulation of steroid receptor function. The proposed research will examine structure-function correlates in the immunophilins Cyp40, FKBP51, and FKBP52 in vitro; use a yeast genetic mutant lacking the homolog of Cyp40 to test for functional complementation by human immunophilins; and examine the function of immunophilins in mammalian cell lines expressing steroid receptors.